Scientific studies, and news coverage about them, often declare that a study “controlled” for certain variables.

For example, the Washington Post reported on a study of GLP-1 drugs and a hormonal disorder. The article noted that “women on GLP-1s lost more weight than those in control groups.”

A couple months earlier, the Post reported on a different study about GLP-1s and bone and tendon injuries. It said “researchers controlled for age, sex, race and tobacco use” and found that people taking GLP-1s had higher risk of skeletal disorders than nonusers.

This language typically gives the impression that having a control enhances credibility. But what exactly does “control” mean? And should you be impressed by it?

I’ve been a statistician for many years, and my recent research has shown that 90% of American adults would base decisions on statistics, at least sometimes, if they understood the basic concepts better. Control is one such concept.

Understanding controls can help you assess how well a study’s conclusions are supported by evidence, and can help you judge the media’s interpretation of new research.

Observed or manipulated?

A control can mean different things depending on a study’s design. Most studies fall into one of two categories: experimental or observational.

In an experimental study, researchers may give a drug or treatment to some participants and not to others. The control group is the people who did not receive the drug or treatment.

In the study of women’s weight loss and hormonal disorders, researchers gave GLP-1s to some of the participants, and did not give them to others, so they could compare outcomes between the two groups. Here, “control”........

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